1. Field of the Invention
The Invention is a sonar dome for use by surface ships or submarines. The sonar dome of the Invention is a composite fiber-reinforced plastic having a low acoustical insertion loss, particularly at high frequencies, coupled with high mechanical strength. The Invention is also a method of manufacture of the sonar dome.
2. Description of the Related Art
Acoustical energy, unlike light and radio energy, may be transmitted through the sea for considerable distances. Modern sonar utilizes acoustical energy for underwater communications, navigation or detection of submerged objects. Passive sonar uses underwater transducers to receive and locate sound generated by, for example, a submerged submarine. Active sonar uses underwater transducers to generate an acoustical signal that travels from the transducer through the sea and is reflected from an undersea object back to the transducer.
A sonar dome is an underwater structure mounted to a vessel, generally at the bow of the vessel. The sonar dome houses the sonar transducers and is flooded to acoustically couple the transducers with the surrounding sea. The purpose of the sonar dome is to protect the sonar transducers from mechanical force exerted by the seawater through which the vessel moves, to protect the transducers from damage caused by contact with objects such as piers, and to reduce the self-noise that would otherwise be generated by turbulent water flow past the transducers. A sonar dome can experience substantial shock loads from pounding of the bow as the vessel travels through rough seas. Mechanical strength therefore is a desirable quality of a sonar dome.
For passive sonar, sound generated by an undersea object passes through the seawater, through the sonar dome and through the water filling the sonar dome to reach the transducers. In active sonar, sound passes through the sonar dome twice—once on its way from the transducers to the undersea object and a second time as the reflected sound returns to the transducers. The acoustical properties of the sonar dome therefore are important to sonar performance.
One important acoustical property of any material is the material's “characteristic impedance.” The characteristic impedance of a material depends in part on the speed of sound through the material and is analogous to electrical impedance of a component in an electrical circuit.
The portion of the sonar dome through with sound passes on its way from and to the transducers is referred to as the “window.” Each location on the sonar dome window has a characteristic impedance, as does the seawater surrounding and filling the sonar dome. When a first material, such as a sonar dome window, meets a second material, such as sea water, and the two materials have dissimilar characteristic impedances, the boundary between the two is referred to as a “discontinuity.”
When a sound wave traveling through sea water encounters the acoustical discontinuity at the boundary of the sea water and the sonar dome window, the sound wave will undergo an abrupt change in direction and speed. Depending on the angle of incidence and the abruptness of the change in speed of the sound wave, part of the acoustical energy will transfer across the discontinuity and into the material composing the window and part of the energy will be reflected back into the sea water adjoining the window. The portion of the sound wave that continues through the material composing the window will be refracted and will change direction.
The greater the difference in characteristic impedance between the sea water and the sonar dome window, the greater the change in direction of the acoustical energy and hence the greater the amount of acoustical energy that will be reflected away from sonar dome window. The greater the reflection of acoustical energy, the less efficient is the sonar system and the less likely the sonar system is to successfully perform its function. Matching the characteristic impedance of sea water as closely as possible is a desirable quality of a sonar dome window.
Periodic structure within a sonar dome window can cause interference effects resulting in phase cancellation and alteration of the wave form of the acoustical energy passing through the sonar dome window. The wavelength of high-frequency sound used for sonar may be as short as 1 mm. In a prior art reinforced plastic sonar dome, phase cancellation effects at short wavelengths may be caused by reflection or refraction of the sound by adjacent reinforcing fibers (such as glass fibers) appearing in reinforcing yarns that are woven into a reinforcing fabric and embedded within a solidified polymer resin. At longer wavelengths, phase cancellation effects can be caused by periodically-located strands of reinforcing yarn. Phase cancellation effects raise the insertion loss, that is, the attenuation of the acoustical energy caused by the sonar dome, and interfere with the wave form of the acoustical signal, reducing the effectiveness of the sonar system.
In a prior art reinforced plastic sonar dome, small air bubbles trapped within the solidified polymer resin can create large acoustical discontinuities due to the substantial difference in characteristic impedance between the air in the air bubbles and the materials from which the plastic sonar dome is constructed. The small air bubbles scatter and reflect sound traveling through the sonar dome, increasing the insertion loss of the sonar dome and reducing the effectiveness of the sonar system.
The prior art teaches sonar domes composed of steel or reinforced plastics. The prior art steel or reinforced plastic sonar domes exhibit good mechanical strength but also exhibit significant differences in characteristic impedance from sea water. The prior art also teaches rubber sonar domes that have characteristic impedances closely aligned with that of sea water, but which have poor mechanical strength. Existing technology rubber sonar domes require reinforcement or pressurization to maintain a proper hull form and to protect the sonar transducers. Reinforcement of prior art rubber sonar domes may be with wire or with fiberglass skins. Prior art rubber sonar domes with fiberglass skins potentially can suffer from delamination problems. Prior art wire-reinforced rubber sonar domes require an autoclave for manufacture, which increases the difficulty and hence cost of manufacture when compared to a reinforced plastic sonar dome. Due to their relatively low stiffness, prior art wire-reinforced rubber sonar domes also require pressurization in use, increasing the potential for failure of the sonar system. The prior art does not teach the sonar dome of the present Invention.